1. Field of the Invention
The present invention relates to a process for purifying flue gases in refuse incineration plants during regeneration of a catalyst which serves for reducing nitrogen oxides.
2. Discussion of Background Information
In various industrial branches, as, for instance in refuse incineration plants, hot flue gases are produced which contain solid and gaseous substances such as dust, flyash, heavy metals, dioxins, furans and also SO2, SO3, NOX, CO and CHX.
In the thermal treatment of waste, the flue gases, for recovery of the heat energy, are cooled in a steam kettle to a temperature of approximately 220° C.-240° C. Before exit into the environment, the flue gases must be freed from pollutants, taking into account legally established limiting values.
Historically, flue gases have been purified by, in a first step, carrying out dedusting by means of electrostatic precipitation or by cloth filters in the form of tubes or pockets. In a second step, the acidic pollutant gases were separated off by a wet or dry purification process. In a last step, denitrification took place. In order to be able to reduce the consumption of reducing agents, frequently catalysts were used (SCR, selective catalytic reduction, processes). The reaction takes place in these processes in the presence of a honeycomb or plate catalyst which is frequently connected within the exhaust gas purification of a refuse incineration plant as follows: a) by means of a high-temperature catalyst downstream of an exhaust gas scrubbing with heating; b) by means of a high-dust high-temperature catalyst downstream of the kettle or c) by means of a low-temperature catalyst as the last stage of exhaust gas purification.
Low-temperature catalysts which generally operate at a temperature range below approximately 220 to 230° C. have the advantage that heating the flue gas is customarily not needed. However, because they operate in the low-temperature range, they are very sensitive, compared with high-temperature catalysts, to salt-type compounds. Therefore, preceding dedusting takes place (low-dust connection) and also removal of acid pollutant gases, in order to prevent blocking and poisoning of active catalyst centers. Since this purification step is not completely successful, low-temperature catalysts must be regularly regenerated.
DE 36 34 360 describes catalyst filters. These were developed to combine four exhaust gas purification stages into one single step in which the catalytic filtration was extended, by addition of adsorbent, for dioxin reduction, and by addition of hydrated lime for dry sorption of acidic pollutant gases. In the filter medium, reactions can occur which are further accelerated by active zones in the filter material. They lead to condensation of ammonium salts, low-melting heavy metal salts and thereby to plugging of pores which can be accompanied by failure of filtration by increase in the pressure drop. EP 1 072 302 describes how such catalyst filters can be regenerated. In this process the catalyst filters are regenerated by simultaneous thermal and mechanical treatment. For this the temperature is elevated by means of a regeneration burner to above 280° C., preferably 320° C., and maintained for some hours. The injection of ammonia is stopped. Although these catalyst filters need not be dismantled for the regeneration process, during the regeneration time, however, refuse charging must be stopped and the combustion chamber must be kept hot using support burners. Customarily, an interruption of functions of approximately 3 to 8 hours must be expected until the regeneration process is complete.
EP 1 576 999 describes a process for purifying flue gases in which the flue gas is filtered, admixed with ammonia and subsequently brought into contact with a catalyst for denitrification. The catalyst is periodically regenerated at temperatures of 280 to 450° C., the catalyst unit to be regenerated being isolated. The purified exhaust gases are passed into the stack. As a result, pollutants released in a thermal regeneration can pass into the atmosphere.